Cast and thermoplastic polyurethane compositions based on the reaction of polyisocyanates with polymeric diols are well known for use as elastomers, adhesives, sealants, elastomeric surface coatings, and coatings for metals and plastics.
The polyurethanes of the present invention are based on a polydiene diol, an isocyanate, and a relatively low molecular weight chain extending diol. In this composition the polydiene serves as the soft, rubbery portion of the polyurethane. The diisocyanate and chain extending diol react together to form a rigid segment of the polymer. Because of the reactivity of isocyanates and alcohols, the polydiene is chemically bound at its termini to the hard segments. In this way a segmented block copolymer is formed.
The strength and other physical properties derive from the molecular structure of the segmented block copolymer polyurethane. In their high molecular weight polymerized form, the soft and hard segments are significantly incompatible and segregate into separate domains. In this arrangement the soft segments act as rubbery polymers and they are physically crosslinked through the segregation and vitrification or crystallization of the hard segments. It is important that the incompatibility between hard and soft segments exist in the final high molecular weight form. Without this incompatibility a leathery material would result which would have limited useful applications due to poor low temperature properties and a low upper service temperature.
While it is important for high molecular weight polyurethanes to have incompatibility between hard and soft segments, this strong incompatibility must not extend to the case of the unreacted components. Intimate contact between reactants is required for full chemical reaction and high polymer formation. If the reactants are incompatible, only low molecular weight polyurethanes are achievable and these have low strengths and are not generally useful.
Thus, the most useful segmented block copolymer polyurethanes are ones in which the reactive components demonstrate compatibility but upon reaction and molecular weight advancement segregate into rubbery soft segments and rigid, reinforcing hard segments. It is this balance of compatibility which must be engineered into this multicomponent polymer which yields the excellent physical properties.
Conventional polyurethanes utilize polyester and polyether soft segments. Low molecular weight chain extenders having a suitable balance of compatibility with these polyols are well known. This invention utilizes a saturated polydiene diol which has significantly different compatibility characteristics. The utility of the saturated polydiene diol relates to its excellent thermal, UV and hydrolytic stability as well as its excellent low temperature mechanical properties. The novel composition herein described is comprised of a particular low molecular weight chain extender which exhibits a suitable balance of compatibility and yields polyurethanes of high strength and hardness.
Polyurethane compositions made with polydiene diols and certain diol chain extenders are described in copending commonly assigned U.S. patent application Ser. No. 08/494,639, filed Jun. 23, 1995, entitled "Use of Polydiene Diols in Thermoplastic Polyurethanes," which is herein incorporated by reference. The compositions described therein contained chain extenders which were low molecular weight diols. The purpose of these chain extenders therein and herein is to increase the level of hard segment (the amount of isocyanate plus the amount of chain extender) in the polyurethane composition. There are many applications for these polyurethane compositions that require high hardness and high tensile properties. One example is ski boots which obviously cannot be soft and pliable. The chain extenders described in the above patent application, both polar and apolar, are not very compatible with the polydiene diol and the isocyanate. In order to make compatible compositions with those chain extenders, the level of hard segment, i.e., the amount of isocyanate and chain extender, must be kept relatively low. This limits the resulting hardness and strength achievable in polydiene based polyurethanes. The patent application describes compositions at 22 percent hard segment and 33 percent hard segment. Even with this limitation, a special solvent process is required to make the polar chain extenders such as 1,4-butane diol sufficiently compatible. This process is expensive and involves solvent removal and recovery. Obviously, it would be advantageous to be able to produce these compositions and others of increased strength and hardness without using the solvent method.
It is an object of the present invention to provide thermoplastic polyurethane compositions which are harder and stronger than currently available TPU compositions made with polydiene diols.